Effectiveness of outpatient and community treatments for people with a diagnosis of ‘personality disorder’: systematic review and meta-analysis

Background Quality of care and access to effective interventions have been widely criticised as limited for people diagnosed with ‘personality disorder’ or who have comparable needs (described in some recent papers as “Complex Emotional Needs” (CEN). It is important to identify effective interventions and the optimal context and mode of delivery for people with CEN. We aimed to investigate the effectiveness of psychosocial interventions delivered in community and outpatient settings in treating symptoms associated with ‘personality disorder’, and the moderating effects of treatment-related variables. Methods We systematically searched MEDLINE, EMBASE, PsycINFO, CINAHL, HMIC, ASSIA for articles published in English, from inception to November 23, 2020. We included randomized controlled trials examining interventions provided in community or outpatient settings for CEN. The primary outcome was ‘personality disorder’ symptoms, while secondary outcomes included anxiety symptoms, depressive symptoms, and global psychiatric symptoms. Random-effects meta-analysis was conducted for each outcome, and meta-regression analysis was performed to assess the moderating effects of treatment characteristics. The quality of the studies and the degree of publication bias was assessed. Results We included 54 trials (n = 3716 participants) in the meta-analysis. We found a large effect size (g = 0.78, 95% CI: 0.56 to 1.01, p < 0.0001) favoring interventions for ‘borderline personality disorder’ (BPD) symptoms over Treatment as Usual or Waitlist (TAU/WL), and the efficacy was maintained at follow-up (g = 1.01, 95% CI: 0.37 to 1.65, p = 0.002). Interventions effectively reduced anxiety symptoms (g = 0.58, 95% CI: 0.21 to 0.95, p = 0.002), depressive symptoms (g = 0.57, 95% CI: 0.32 to 0.83, p < 0.0001), and global psychiatric symptoms (g = 0.50, 95% CI: 0.35 to 0.66, p < 0.0001) compared to TAU/WL. The intervention types were equally effective in treating all symptom categories assessed. Treatment duration and treatment intensity did not moderate the effectiveness of the interventions for any outcome. Conclusions People with a ‘personality disorder’ diagnosis benefited from psychological and psychosocial interventions delivered in community or outpatient settings, with all therapeutic approaches showing similar effectiveness. Mental health services should provide people with CEN with specialised treatments in accordance with the availability and the patients’ preferences. Supplementary Information The online version contains supplementary material available at 10.1186/s12888-022-04483-0.


Introduction
Difficulties associated with a 'personality disorder' diagnosis are common and debilitating, with the prevalence estimated to be 7.8% worldwide [1]. Reflecting the concerns around the diagnostic validity of the diagnostic term 'personality disorder' and the potentially stigmatising effect for those receiving this diagnosis [2][3][4], alternative terms, such as Complex Emotional Needs (CEN), have been developed to describe this condition [5]. In this review, the terms CEN and 'personality disorder' will be used interchangeably.
People diagnosed with 'personality disorder' experience severe impairment and distress from adolescence or early adulthood onwards, affecting various aspects of their life, including social and vocational functioning as well as mental and physical health [6][7][8][9][10][11]. Additionally, people with CEN experience poor well-being [12], lower quality of life [13], and reduced life expectancy [14], including an increased risk of dying from homicide, suicide, or accident compared to the general population [15]. Lastly, a diagnosis of 'personality disorder' is associated with high costs through high utilisation of healthcare systems, leading to increased economic and societal burden [16,17].
Recent evidence suggests that the difficulties associated with a 'personality disorder' diagnosis change substantially over time, especially following psychological or psychosocial treatment targeting the core symptoms [18]. There is a consensus that psychological interventions are the first-line treatment for people diagnosed with 'personality disorder' [19,20]. Α number of reviews and meta-analyses have found evidence that some types of community interventions, especially psychodynamically informed treatments and cognitive and behavioral therapy (CBT), are effective in treating 'personality disorder' symptoms [21][22][23][24][25][26].
The majority of studies have focused on 'BPD' and have shown that specialised psychotherapeutic interventions, such as psychodynamic therapy, dialectical behaviour therapy (DBT), mentalization-based therapy (MBT) and transference-focused therapy (TFT), are efficacious for this condition [27][28][29][30][31][32][33][34][35]. A recent Cochrane review found that a wide range of psychological interventions could effectively reduce 'BPD' symptoms, alongside other outcomes [32]. However, previous reviews have been limited in their inclusion criteria, focusing on samples with 'BPD' diagnosis, without examining people diagnosed with other 'personality disorders' , such as 'antisocial personality disorder' , 'narcissistic personality disorder' or cluster A and C 'personality disorders' or those reporting symptoms associated with CEN.
While a number of people experiencing CEN receive treatment in outpatient settings, including day hospitals, a substantial proportion of patients, predominantly those with a 'BPD' diagnosis, are admitted to inpatient hospitals where they receive short-term crisis interventions [36,37]. Although inpatient treatment may sometimes be necessary and effective in treating acute symptoms associated with CEN [36,38], the current focus is on the provision of interventions in community mental health care settings, including outpatient facilities, such as day hospitals, as the latter are beneficial for the establishment of long-term therapeutic outcomes [39], and cost effective [40].
The lack of definitive evidence regarding the optimal treatments for people with CEN has led to large heterogeneity in the guidelines across different countries [41], and the services provided for this population [42]. A further impediment lies in the focus of the guidelines on 'BPD' diagnosis, with only a few treatment recommendations for people diagnosed with other 'personality disorders' or experiencing associated experiences (e.g., repeated self-harm or suicide attempts, complex trauma or complex post-traumatic stress disorder (c-PTSD) [41]. Lastly, only limited evidence exists on the impact of the treatments' qualities and the context that those are being provided (e.g., day hospital, generic mental health services), in the effectiveness of the Interventions.
In light of the above limitations, the aim of this systematic review and meta-analysis is to: a) examine the effectiveness of psychological and psychosocial interventions delivered in community settings in treating 'personality disorder' symptoms, as well as depressive, anxiety, and global psychiatric symptoms, b) compare the effectiveness of different intervention types for the outcomes above, and finally c) investigate the moderating effects of treatment characteristics in the effectiveness of the interventions, and the optimal context of delivery.

Methods
This study was developed following PRISMA guidelines [43], and a protocol was registered on PROSPERO (reference: CRD42019143165). The current review was part of a programme of work that included four individual systematic reviews. A single search strategy was used for the whole programme. The protocol for the wider programme of work was also registered on PROS-PERO (CRD42019131834). The current meta-analysis has its basis on a larger scoping review [44]. The extent of heterogeneity in the literature (e.g., samples included, outcomes assessed) led to the decision to conduct metaanalyses on a more limited subset of the data, including higher quality data from randomized control trials (RCTs) only.

Search strategy and selection criteria
To identify eligible articles for this systematic review and meta-analysis, a search strategy was developed for the following bibliographic databases: MEDLINE, EMBASE, PsycINFO, Cumulative Index to Nursing and Allied Health Literature (CINAHL), Social Policy and Practice, Health Management Information Consortium (HMIC), and Applied Social Sciences Index and Abstracts (ASSIA). The original search, developed in collaboration with a project-specific working group that included people with lived experience of CEN, was conducted in December 2019. An update search was carried out in November 2020. The search encompassed terms relating to CEN, community or outpatient settings, and psychological or psychosocial treatments. Websites of known relevant organisations were also searched. Lastly, reference lists of all included studies were hand-searched, and reference search of relevant systematic reviews was conducted to identify further studies. Full details of the search strategy are shown in the Additional file 1 (Tables S1-S6).
Studies meeting the following criteria were included in this review: Population: Adults (90% of the sample over 16 years old or a mean sample age of 18 or over) in which a majority (> 50%) was diagnosed with 'personality disorder' or participants identified as experiencing symptoms or difficulties related to a diagnosis of 'personality disorder' (i.e., repeated self-harm, emotion dysregulation or instability).
Intervention: Psychological or psychosocial interventions conducted in a community mental health care setting, including a wide range of outpatient facilities, such as day hospitals. Studies were included if participants were provided with any kind of group or individual psychological or psychosocial intervention, such as DBT, psychodynamic treatment, MBT or emotional regulation programmes. Interventions should primarily target CEN and follow a protocol developed for this population. We excluded interventions carried out in forensic, crisis care or inpatient settings.
Comparator: Eligible controls included TAU, standard care, WL or no intervention, or alternative types of active treatment(s).
Outcomes: Eligible outcomes included 'personality disorder' symptom severity or 'BPD' symptom severity, anxiety symptoms, depressive symptoms or psychiatric symptoms measured on a validated scale. In the instances that more than one rating scale was used to examine a specific outcome, one was chosen, based on the psychometric properties and the frequency of use in other included studies. Outcomes measured by subscales were excluded from the analysis, leaving only full scales developed for the specific outcomes.
Study design: Only RCTs were included in this review. Studies with alternative designs which met other criteria were included in our scoping review, which did not include a meta-analysis [44].
We excluded studies whose primary focus of treatment was not 'personality disorder' or associated needs. We also excluded theses and conference abstracts. Only studies published in English were included. The full search and screening process is depicted in Fig. 1.

Study selection
After removing duplicate records, titles and abstracts were independently screened by one researcher, with 10% of them being double checked by a second researcher. The full texts of those that appeared eligible were examined independently by two researchers. Discrepancies or disagreements were resolved through discussion with the research team.

Data extraction and quality assessment
Data were extracted by the same reviewers using a custom extraction form on Microsoft Excel and were double checked for accuracy. The standardised extraction form included study characteristics (first author name, title, year of publication, country, setting), intervention details (control category, type of treatment, treatment intensity, treatment duration), patients characteristics (number of participants, diagnostic tool), and outcome details (type of outcome, measure, measurement timepoints, effect sizes). Regarding the outcomes, in cases where more than one measures was used to assess a specific outcome (e.g., 'BPD' symptom severity) one of them was retained and analysed, based on the validity and frequency of use in other relevant studies. For the selection of the outcomes of interest in this meta-analysis, we consulted experts in the field (e.g., SP).
The quality of individual RCTs was assessed using the Cochrane risk of bias tool [45]. This tool provides a framework for considering risk of bias in the findings of any type of randomized trial and is structured into six domains through which bias might be introduced into the result (randomization, selection, performance, detection, attrition, reporting). Quality assessments of 10% of the studies were checked for accuracy and correct application of the tool. Any uncertainty about ratings was resolved through discussion with a senior researcher.

Data synthesis and analysis
For the meta-analysis, effect sizes statistics were calculated as standardized mean difference (SMD), using the metafor package of the R software [46]. The latter automatically corrects the positive bias in the SMD, providing Hedges g [47]. Hedges g pools variances and standardizes outcomes across studies which allow for comparison among disparate outcome measures. Calculations used a random-effects model, which assumes that analyzed studies represent a random sample of effect sizes, facilitating generalizability [48]. Given that the studies included used different populations with differing inclusion specifications, this statistical model was considered appropriate. For the assessment of Heterogeneity, I 2 statistic was calculated. A value of 0% tentatively indicates the absence of heterogeneity, and 25, 50%, or 75% signifies low, moderate, or high heterogeneity between studies, respectively [49]. Data for each of the outcomes, namely 'personality disorder' symptoms, 'BPD' symptoms, anxiety symptoms, depression symptoms, and psychiatric symptoms were analyzed in separate analyses. Active and non-active (TAU, WL, or no intervention) controls were also analyzed separately. Outcomes were grouped into categories according to the time point post-intervention they were measured: End of treatment (EOT), 1-7 months, 7-12 months, 13-18 months, and over 18 months followup. Subgroup analyses were also conducted to compare different intervention types for the outcomes above. Studies investigating interventions that could not be classified in any of the major intervention categories were assessed individually in a narrative synthesis. Analyses were performed on any category with at least K = 2 interventions. Where studies did not report outcomes at EOT but provided a follow-up of 1 month or less from EOT, this was examined as the EOT measure. A p value of < 0.05 was considered to be statistically significant and the following conventional values of effect size for SMD were used [50]: an effect size of 0.2 signifies a small, 0.5 a moderate, and 0.8 a large effect.
We conducted meta-regression analysis to examine the potential moderating effect of the following variables on the effectiveness of the interventions: intervention category (CBT, psychodynamic treatments, MBT, DBT, Schema Therapy), service setting (specialist day service, specialist team, standalone outpatient intervention, generic mental health service), diagnoses ('BPD' , Other 'personality disorders -studies assessing either a non-BPD 'personality disorder' or more than 1 'personality disorder' , CEN symptoms without a diagnosis of 'personality disorder' (e.g., self-harm or complex trauma), intervention intensity (< 2 sessions, 2-3 sessions, > 3 sessions per week) and intervention duration (< 4 months, 4-6 months, 7-12 months, > 12 months). Given that only a limited number of studies included participants with a diagnosis of 'personality disorder(s)' other than 'BPD' , those were grouped together and were compared to studies with BPD-only sample. Meta-regressions were conducted where at least K = 10 studies examined the outcome for a specific timepoint [51]. As such, metaregressions were performed for each moderator for 'BPD' symptoms, anxiety symptoms, depressive symptoms, and psychiatric symptoms at EOT. The degree of publication bias for each outcome was assessed by visual examination of the funnel plot and by conducting Egger's regression test [52].

Results
After our initial search (inception-December 2019), 17,511 studies were obtained, and 12,126 records were identified for screening. From there, we identified 526 potentially eligible studies for full text screening. After excluding 482 studies, 44 studies were included. A further three studies were found from backwards reference search and forwards citation search and four were detected from existing relevant systematic reviews. In a second updated search that aimed to identify studies from December 2019 to November 23rd, 2020, 1868 studies were obtained, and 1328 new records were identified for screening. Of those, 34 potentially eligible studies were identified for full-text screening. Two studies met the eligibility criteria and were added to the review. After searching relevant systematic reviews, one further study was included. Overall, 54 RCTs examining community treatments for 'personality disorder' were included in this review, and 57 comparisons between intervention and control group were meta-analysed .

Study characteristics
The characteristics of the included studies are summarized in Table 1. In total, this review included 3716 participants, who received any type of community treatment for personality disorder. CBT was the most common intervention investigated, examined in 14 trials. Eleven trials examined psychodynamic therapies, nine examined DBT, eight assessed MBT, two investigated Schema Therapy, and 13 studies could not be grouped in any of the aforementioned intervention types and were collectively categorized as 'Other Treatments' . The latter treatment category included either less prominent types of psychotherapy that could not be categorized in the treatment types studied (e.g., TFP), or treatments focusing on social or global outcomes, such as Nidotherapy. Forty-two studies included a non-active comparison control group,  while 12 studies compared the interventions to an active control group, which received a specialist intervention. Thirty-two studies included interventions delivered as standalone outpatient interventions, seven studies included interventions that were provided in specialist day services, in seven studies interventions were offered in generic mental health services, and in seven studies interventions were delivered by a specialist team. Thirty-seven studies included participants diagnosed with 'BPD' , while 16 studies included participants diagnosed with a specific 'non-BPD' 'personality disorder' (e.g. 'avoidant personality disorder') or a sample of participants diagnosed with different 'personality disorders' . One study included participants with either a diagnosis of 'personality disorder' or presenting personality difficulties without a formal diagnosis. Most studies (K = 38) were conducted in European countries, 13 were conducted in North America, one in Australia, one in Iran, and one included two sites, one in Norway and one in Canada.

Quality assessment and publication bias
Overall, the risk of bias examination indicated that the quality of the studies was low to moderate, although large variations were observed between studies (Additional file 2, Fig. S1). The majority of the studies reported adequate random sequence allocation, while fewer reported adequate allocation concealment. All included studies were assessed as having high performance bias, something common in complex psychosocial interventions, where therapists and participants cannot be blinded. More than half of the studies displayed high risk of detection bias, as most of the outcomes were measured using self-report measures. Around one quarter of the studies referred to a protocol that could be found online, while the rest did not report having one. More than three quarters of the studies were assessed as having low risk of other bias, that would emerge from broader methodological errors or omissions.
The funnel plots (Additional file 3, Fig. S2) showed that there is considerable asymmetry for 'BPD' symptoms, depressive symptoms, and psychiatric symptoms, indicating publication bias. The results from the Egger's test confirmed the absence of publication bias for anxiety symptoms (p = 0.12) and the presence of publication bias for 'BPD' symptoms (p = 0.02), depressive symptoms (p = 0.02), and psychiatric symptoms (p = 0.05).

'Personality disorder' symptoms
Two studies assessing general personality disorder symptoms at > 18 months follow-up found weak evidence in favor of the interventions compared to TAU/WL (g = 0.25, 95% CI: − 0.05 to 0.55, p = 0.10, I 2 = 50.49%), without reaching statistical significance. Three studies found no difference in personality disorder symptoms between the interventions under investigation and the active control interventions at > 18 months follow-up (g = − 0.13, 95% CI: − 0.45 to 0.19, p = 0.43, I 2 = 26.83%).

Depressive symptoms
Overall, 23 comparisons compared interventions to control conditions for depressive symptoms at EOT. Sixteen comparisons comparing interventions to TAU/WL demonstrated a medium effect size (g = 0.57, 95% CI: 0.

Narrative synthesis
Thirteen studies were included in the Other Treatments category, and a brief narrative synthesis was conducted on the efficacy of each of the interventions compared to control comparisons.
Two interventions were aimed at the reorganization of the services for people with personality disorder.
Stepdown treatment was not effective in reducing personality disorder symptoms, depressive symptoms, and psychiatric symptoms compared to outpatient therapy [56,57]. In contrast, Democratic Therapeutic Community, a form of psychosocial treatment based on a collaborative approach to staff-patient interaction, significantly reduced the severity of psychiatric symptoms (g = 0.72, 95% CI: 0.07 to 1.38) compared to TAU [98].
Three studies examined interventions designed to enhance self-management and provide care. In a onesession intervention study, Joint Crisis Plans did not reduce anxiety symptoms and depressive symptoms at 6 months follow-up compared to TAU [65]. Another single-session trial found that therapeutic assessment was not superior compared to goal-focused pretreatment intervention in reducing psychiatric symptom severity [73]. Finally, a trial comparing Nidotherapy and TAU for psychiatric symptom severity, found that both interventions were equally effective for this outcome [101].
Another study that compared Sequential Brief Adlerian Psychodynamic Psychotherapy plus supervised team management and supervised team management alone found no difference between the two interventions in reducing psychiatric symptoms [54]. Abandonment psychotherapy was found to reduce the severity of depressive (g = 0.59, 95% CI: 0.19 to 0.99) and psychiatric symptoms (g = 0.81, 95% CI: 0.41 to 1.22) compared to TAU [55]. In a study it was found that TFP did not reduce depressive, anxiety, and psychiatric symptoms compared to TAU, however, it could significantly reduce 'BPD' symptoms (g = 0.55, 95% CI: 0.16 to 0.95) [74]. Another trial found no difference between Community Treatment by Experts and TAU in treating 'BPD' symptoms [93]. Brief adaptive therapy could significantly reduce psychiatric symptoms (g = 1.27, 95% CI: 0.56 to 1.97) compared to WL at EOT [106].
A trial comparing the effectiveness of cognitive rehabilitation and psychoeducation on anxiety, depression, and 'BPD' symptoms, found that both interventions yielded comparable effectiveness [97]. Another study found that 10 sessions of art therapy were effective in reducing psychiatric symptom severity at EOT (g = 1.23, 95% CI: 0.66, 1.79) and 5-week follow-up (g = 1.35, 95% CI: 0.77 to 1.92) compared to waitlist [83].

Meta-regression analysis
The results indicated that there was no significant association between the types of intervention (CBT, Psychodynamic Treatments, DBT, MBT & Schema Therapy) and intervention effectiveness for any of the outcomes examined (Table 3). Meta-regressions for the moderating effect of service setting found no significant association. However, the results showed that there is significant association between diagnoses and the effectiveness of the interventions for depressive symptoms. Studies including participants with any 'personality disorder' reported worse outcomes after treatment compared to studies including a 'BPD' only sample on depressive symptoms (b = − 0.71, 95% CI: − 1.39 to − 0.03, p = 0.04). Diagnosis was not associated with the effectiveness of the interventions for the other outcomes examined.
In terms of treatment duration, the results were close to achieving statistical significance, supporting the association between treatment duration and effectiveness of interventions on anxiety and depressive symptoms. Participants who received treatments that lasted 7-12 months reported worse outcomes in anxiety symptoms (b = − 0.80, 95% CI: − 1.61 to 0.01, p = 0.05) and depressive symptoms (b = 0.55, 95% CI: − 1.10 to 0.01, p = 0.05) compared to those receiving less than 4 months of treatment. Moreover, treatments that lasted 4-6 months and 7-12 months were less effective in reducing 'BPD' symptom severity compared to treatments lasting less than 4 months (b = − 0.59, 95% CI: − 0.98 to − 0.20, p = 0.003 and b = − 0.50, 95% CI: − 0.90 to − 0.11, p = 0.01, respectively). We performed additional meta-regression analyses, where treatment duration was included as a continuous moderator. In this case, the moderating effect of treatment duration ceased to be significant for 'BPD' symptoms (b = − 0.01, 95% CI: − 0.04 to 0.02, p = 0.56), depressive symptoms (b = 0.01, 95% CI: − 0.03 to 0.04, p = 0.79) and anxiety symptoms (b = − 0.01, 95% CI: − 0.06 to 0.05, p = 0.82). Figure 4 illustrates the bubble plot of the regression line for 'BPD' symptoms. No association between treatment duration and the effectiveness of the interventions for psychiatric symptoms was detected. Treatment intensity was not associated with the effectiveness of the interventions for any of the assessed outcomes.

Discussion
This systematic review and meta-analysis aimed to examine the effectiveness of a wide range of interventions provided in community settings or outpatient facilities for people with a diagnosis of 'personality disorder' . This review is the first to systematically examine trials conducted on participants with a 'personality disorder' diagnosis beyond 'BPD' and people without a formal diagnosis but relevant symptoms and examine a variety of outcomes not previously investigated in this population. Moreover, multiple moderators that could potentially impact the efficacy of the interventions were examined. In total, 54 RCTs were included in this review. The results indicated that community interventions were effective in treating 'BPD' symptoms, anxiety symptoms, depressive symptoms, and global psychiatric symptoms compared to non-active comparisons at EOT. The effectiveness of the interventions was maintained longer term (7 months after the end of treatment) for some outcomes, particularly for psychiatric symptoms and 'BPD' symptoms. There was weak evidence supporting the efficacy of the interventions in reducing general 'personality disorder' symptoms and promoting recovery at follow-up.  The results from meta-regression analysis indicated that community treatments are effective for the outcomes assessed, regardless of the type of intervention investigated (e.g. CBT, Psychodynamic treatments, DBT, MBT and Schema Therapy). More specifically, CBT, which was the most intensively studied treatment, psychodynamic treatments and DBT were found to significantly ameliorate an array of outcomes that were assessed. Although it was not possible to conduct a meta-analysis due to the limited data, individual studies examining MBT [60][61][62]88], and Schema Therapy [58,77], indicated that both treatments are effective in reducing 'personality disorder' symptoms. Other treatments, such as Democratic Therapeutic Community treatment [98], abandonment therapy [55], and brief adaptive therapy [106] were also found to be beneficial for people with CEN. However, further trials need to examine the efficacy of those interventions in the future. Interestingly, a trial found that Art therapy [83] was effective in treating global psychiatric symptoms in this population. Accumulative evidence suggests that art therapy can mitigate 'personality disorder' symptoms and increase well-being in people with CEN through different mechanisms [107].
The results from this meta-analysis are consistent with the findings from the recent Cochrane systematic review [32], regarding the efficacy of psychological interventions for 'BPD' symptom severity and depressive symptoms for people diagnosed with 'BPD' . However, the present study included not only psychological interventions but also treatments provided as part of a day-treatment programme (e.g. Democratic Therapy) or other psychosocial interventions (e.g. Art Therapy) and included studies with participants diagnosed with 'personality disorder(s)' beyond 'BPD' . Also, the current review examined anxiety symptoms and general psychiatric symptoms as outcomes as well, providing evidence for the efficacy of the interventions for additional outcomes. The results from this study also partially support the findings from a previous review examining psychotherapies for 'BPD' [27], which found that psychotherapies, particularly DBT and psychodynamic approaches were effective in treating 'BPD' symptoms. However, our review found larger effect sizes for the efficacy of DBT and psychodynamic therapy for 'BPD' symptoms, while, in contrast with the former review we found CBT to be effective for this specific outcome. This divergence in the findings might reflect the characteristics of the studies included in this review (e.g., different populations and outcome measures for 'BPD') and the control comparisons of the interventions which were assessed. The effectiveness of specialized psychotherapies (DBT, MBT, TFP and Schema Therapy) was also reported by a recent systematic review [31], which found that psychotherapies are effective in reducing 'BPD' symptoms. This study adds to the previous evidence suggesting that psychological and psychosocial interventions delivered in community and outpatient settings are not only highly effective in treating 'BPD' symptoms but can also improve a wide range of psychiatric symptoms in this population, with considerable maintenance across time. This is in line with previous evidence suggesting that 'BPD' symptoms are strongly associated with an array of psychiatric symptoms comprising the general psychopathology factor [108], thus raising the need to deliver interventions targeting global symptomatology, beyond 'BPD' symptoms.
The efficacy of CBT for anxiety and depressive symptoms compared to TaU/WL is in line with considerable literature supporting the effectiveness of this intervention for the aforementioned symptoms [109][110][111]. This is the first study to systematically examine those outcomes for people with CEN. Previous reviews have also found that CBT can effectively treat a wide spectrum of psychiatric conditions [112,113], including 'BPD' symptoms [28]. The effectiveness of psychodynamic treatments for people diagnosed with 'personality disorder' has also been documented in the literature. Recent reviews have found that psychodynamic treatments are superior to control groups in treating 'personality disorder' symptoms and general psychiatric symptoms in people diagnosed with 'personality disorder' [26,27]. This review expands on existing literature by supporting the efficacy of this intervention for global psychiatric symptoms manifested by patients with CEN, as well as for depressive and 'BPD' symptoms. The effectiveness of DBT for 'BPD' is well evidenced in the literature [27,114]. This review adds to the previous studies suggesting that this intervention can be effective for other psychiatric symptoms (e.g., anxiety symptoms) in people with CEN. It is possible that improvements in the core symptomatology of 'personality disorder' lead to the reduction of the severity of other psychiatric symptoms (e.g., depression and anxiety). Although meta-analysis could not be conducted for studies on MBT, the effectiveness that emerged from two trials is consistent with previous reviews [34]. However, more studies are needed for firm conclusions to be drawn on the efficacy of this intervention. Finally, although the number of studies was small and meta-analysis could not be conducted, accumulative evidence suggests that Schema therapy is beneficial for people with a diagnosis of 'BPD' and those being diagnosed with cluster C 'personality disorders' or 'narcissistic personality disorder' [58,77,115]. Although the effectiveness of the major psychotherapeutic types delivered to people experiencing CEN has been widely investigated, trials examining treatments for people with CEN and major psychiatric comorbidities (e.g. Depression) are limited, while studies for specific age groups (e.g., older patients) have been rarely conducted.
The results from the meta-regression analysis provide insight into the moderating effects of different factors that can potentially impact the effectiveness of treatments for an array of outcomes, something that has not been investigated by previous studies. The equal effectiveness of the psychotherapeutic approaches examined is consistent with previous studies that could not detect significant differences between treatment types in reducing 'BPD' symptoms or depressive symptoms [28,31,32], and provides new evidence for other psychiatric symptoms as well. Interestingly, interventions delivered to participants with any 'personality disorder' diagnosis were less effective in treating depressive symptoms compared to 'BPD'only studies, while there was weak evidence for anxiety symptoms. This potentially reflects the focus of the existing treatments on 'BPD' diagnosis, while specific protocols for other 'personality disorder' diagnoses (except for 'antisocial personality disorder') have not been developed. Furthermore, it could be possible that 'personality disorders' other than 'BPD' are more difficult to treat with the existing interventions. In terms of treatment length, our results showed that treatments with differing duration were equally effective for the outcomes examined. This contrasts with previous evidence supporting the superiority of longlasting interventions in treating 'BPD' symptoms [25], and sheds light on the ongoing debate concerning the importance of treatment length in treating 'personality disorders' . Given the high symptom severity that people with 'BPD' may initially present with, it is possible that the first few treatment sessions lead to significant symptom reduction and stabilisation, effects that subsequently plateau in the course of the treatment. Consequently, short-term interventions can potentially be advantageous compared to long-term interventions, as they might have the same therapeutic effect while leading to reduction in health service costs and clinician time. However, given the contradicting nature of our findings, the latter should be treated with caution, and further well-designed studies comparing duration of treatment are required to draw firm conclusions regarding the role of treatment length in intervention effectiveness.
Although treatments delivered in the community can effectively treat 'personality disorder' symptoms, alongside global psychopathology, less is known about the ability of those treatments to change core personality and improve functioning. While some evidence suggests that personality traits can change following intervention [116], studies examining personality change as a treatment outcome in people with 'personality disorder' diagnosis are limited. In addition, little evidence is available regarding the efficacy of psychological and psychosocial treatments in improving functioning in this patient group. Therefore, we propose a shift in research towards incorporating additional treatment outcomes, such as those stated above.

Limitations of the study
This study has several limitations. First, the majority of the studies were conducted on patients diagnosed with 'BPD' . A consequence is that most trials included mainly female participants, with only a few RCTs including a comparable number of male participants. Also, participants of ethnic minority groups were underrepresented in the samples examined. Second, the quality of the studies was generally low to moderate, indicating high risk of bias in some cases. The latter might have had an impact on the effect estimate, leading to the overestimation of the effect sizes [117,118]. In addition, indications of publication bias were present for most outcomes. Third, high heterogeneity was evident in some of the meta-analyses, although steps were taken to try and reduce this through separate examination of interventions and control comparisons. Our exploration of heterogeneity also did not show significant influence of a number of treatment factors, suggesting that participant characteristics may instead play a key role. Most studies included small samples, which can potentially lead to sample error [119]. It should be noted that non-English studies were excluded from this study, which might have introduced bias [120]. Furthermore, although our analysis was pairwise, comparing each intervention to a control condition, there is the possibility that variations in baseline symptom severity moderated the outcomes. Another potential limitation is the exclusion of non-randomized studies. Although this was decided to ensure that high-quality studies are included in the meta-analysis, other study designs can provide significant and clinically meaningful evidence. Our recent scoping review, which examined the treatments available for CEN, included studies with multiple study designs [44]. Lastly, a number of studies were excluded for testing modified or partial forms of already established psychotherapies (e.g., DBT). Although this significantly reduced the heterogeneity within the intervention categories, the efficacy of the former psychotherapeutic interventions was not assessed in this meta-analysis. In line with our recent scoping review [44], only a limited range of interventions were identified in the literature for this population, especially of trauma-informed treatments.

Clinical implications
This study has significant implications for clinical practice. The results indicate that interventions are superior to TAU or no treatment in treating 'personality disorder' symptoms and other symptoms associated with this diagnosis. Therefore, people with CEN should be provided with ready access to specialized community treatment. Given the comparable efficacy of CBT, DBT, and psychodynamic treatments, all could be provided in community settings, while further research investigating which treatments work for which people in which settings would be valuable. The current evidence suggests that Schema Therapy could be offered for people diagnosed with cluster B and cluster C 'personality disorder' , and MBT could be provided to people diagnosed with 'BPD' . However, further evidence is needed to establish the effectiveness of the latter interventions. Structured and time-limited treatments, such as CBT, besides being effective for several psychiatric conditions, are also efficient. Given the effectiveness of this therapeutic approach in treating 'personality disorder' symptoms, CBT can be offered as a first-step intervention in primary care settings. Complex cases, or those not responding to CBT, could be referred to secondary care services, which can provide high-intensity and specialized interventions. Further development and testing of stepped-care intervention models would also be valuable for this population. This would involve informing primary care practitioners about the difficulties people who receive a 'personality disorder' diagnosis may experience and providing training in the assessment and initial treatment of this condition. Lastly, services should reorganize their facilities to meet the patients' needs by providing them with more autonomy and decision making, informing them about each intervention's qualities and characteristics (i.e. duration, intensity, treatment style, working model), and enabling them to choose between treatments that show similar effectiveness [98].

Future directions
In terms of future research, further high-quality trials with larger samples need to be conducted, which will measure outcomes at different follow-up time points to investigate long-term effectiveness. Some suggestions for improving the overall quality of the trials are the blinding of the assessors, the use of genuine randomization methοds (e.g., permuted block randomization), the use of appropriate and clear subject selection criteria (e.g., established diagnosis of 'personality disorder' and examination of potential comorbidities), and the adherence to a published intervention protocol. Importantly, valid scales measuring core 'personality disorder' symptoms and other psychiatric symptoms, should be used universally for the measurement of each outcome to facilitate the comparison of intervention effects between trials. Another significant methodological necessity would be to develop and consistently use standardized control comparison interventions (e.g. manualized supportive therapy) in trials examining psychological interventions, in order for the comparisons to be comparable across different studies. Researchers who conduct each study should not be involved in the delivery of any treatment and instead use independent clinicians or therapists. The examination of existing interventions for different types of 'personality disorders' is necessary to shed light on potentially effective therapeutic interventions for 'personality disorders' beyond 'BPD' . Moreover, future trials should investigate: a) the effectiveness of understudied psychosocial interventions, such as peer support and of different approaches to designing services, b) the effectiveness of different interventions across populations, including younger and older people and people of minority ethnic groups, and c) outcomes valued by service users, such as loneliness, occupational status, quality of life, and treatment targets that matter to them (e.g., trauma). Further developments of existing interventions involving the contribution of service users is also essential for the improvement of the treatment that the latter group receives. Finally, more research is needed to explore the mechanisms of change [19,121].

Conclusions
In conclusion, this systematic review and meta-analysis provides evidence that psychological and psychosocial treatments offered in community or outpatient settings are beneficial for people experiencing 'personality disorder' symptoms and can significantly reduce a range of psychiatric symptoms, including symptoms of 'BPD' . Community mental health practitioners can utilize the psychological interventions available to treat people showing symptoms associated with a 'personality disorder' diagnosis. Mental health guidelines need to highlight the effectiveness of treatments provided in the community for this population, while stepped-care interventions should be developed and delivered in community settings. Challenging therapeutic pessimism associated with this patient group is equally important, considering the effectiveness of the interventions and patients' engagement with those treatments.